In communication systems a modem is used to convert (modulate) digital signals generated by a computer into analog signals suitable for transmission over telephone lines. Another modem, located at the receiving end of the transmission, converts (demodulates) the analog signals back into digital form. Transmitter and receiver circuits located in a central switching office linecard are generally maintained in a power-on state waiting for communication requests from a user. The central office linecard is also used to store system parameters such as equalizer coefficients or automatic gain control (AGC) settings in the case of a sudden drop of signal power or other interruptions. The linecard must discriminate between a desired received signal and noise or other interference signals based on the level and the occupied frequency range. in addition, on the customer side (CPE-customer premise equipment), the receiver must be able to perform a similar task.
The advantage of maintaining the central office equipment in a power-on state is that it will respond quickly to users'requests. However, this results in a significant waste of power when the receiver is inactive. Traditional simple analog energy or peak detect methods for signal detection are easily triggered by impulse noise or other radio frequency (RF) signals.
Further, in situations where the received signal disappears suddenly due to a power surge or other transmitter failure, prior art systems would start to reconverge their equalizer and timing recovery circuits on a non-QAM (quadrature amplitude modulation) signal, which led to problems when the proper QAM signal reappeared.
Consequently, there is a need for an energy detection system that permits the receiver and transmitter circuits in modems to conserve power when there is no communication requested by a user, and adapted to detect a valid user signal reliably and differentiate it from high level noise or other types of interference.